A hydrogel comprises groups of 1,2-diol or 1,3-diol acetalised with a radiopaque species. In particular, the hydrogel may form microspheres, comprise polyvinyl alcohol (PVOH, PVA), and/or the acetal which is formed may be a cyclic acetal. Preferably radiopacity is achieved by coupling a radiopaque moiety having covalently attached halogens, such.as iodine through a cyclic acetal. Such microspheres can be loaded with drugs or other therapeutic agents to provide an imageable drug delivery system.
